CCD pile-up model used for brightish
point sources observed by Chandra. This is an implementation of the fast
pile-up algorithm proposed by John Davis (see http://space.mit.edu/~davis/papers/pileup2001.pdf).
The frame time and maximum number of photons to pile up should be fixed. The
grade morphing is expressed through a single parameter, alpha, which should be
left as a free parameter. This model should be considered in beta test. Note
that to calculate fluxes etc. for the model you must remove the pileup
component. The pile-up model is similar to the operation of the convolution
models, differing only in the treatment of the detector efficiency during the
convolution. Note that renorm will not work with pileup since
increasing the normalization does not linearly increase the predicted count
rate. Therefore you should set renorm none prior to doing a fit with pileup.

par1

frame time (in seconds)

par2

maximum number of photons to
pile up

par3

grade correction for single
photon detection

par4

grade morphing parameter
(good grade fraction is assumed proportional to par4p-1 where p is the number of
piled photons)

par5

PSF fraction. Only this
fraction will be treated for pile-up. Note that this is not the
fraction of the PSF included in the extraction region but is the fraction of
counts in the region which are from the point source whose pile-up is being
modeled. For this model to work well the extraction region should be large
enough to contain essentially all the PSF.

par6

Number of regions. The counts
to be piled-up will be distributed among par6 regions, which will be piled-up
independently.